The present invention is directed to an improvement in an amplifier of the type which includes at least a drive stage and an output stage and, more particularly, to such an amplifier wherein a field effect transistor is included in the output stage.
A typical type of amplifier which is used for audio signals is comprised of a drive stage and an output stage directly coupled to the drive stage. That is, the coupling between the drive and output stages is not frequency sensitive and, typically, may comprise merely a conductor, a resistor or a bias source. Hence, both DC and AC signals can be applied through this direct coupling. In amplifiers of this type, the final element in the drive stage is comprised of an emitter-follower transistor device, and this device is direct coupled to, for example, the gate electrode of a field effect transistor (FET) which is included in the output stage, and which generally is connected to the load which is driven. In one example, the load is driven by a push-pull output stage formed of two complementary FET's; and these FET's are driven by complementary emitter-follower transistor devices. Thus, the FET's may be driven in complementary symmetry that permits push-pull amplification, advantageously without the use of a transformer.
While the emitter-follower devices in the drive stage may be formed of complementary transistor devices so as to properly drive the FET's, the emitter loads for these emitter-follower devices generally consist of a load resistor. Since the FET's are driven in complementary symmetry whereby first one and then the other is turned on, or rendered conductive, this resistor load provides a current path for the gate leakage current of that particular FET which is turned off. This leakage current is on the order of several to ten microamps. With this current flowing through the emitter load resistor, a corresponding voltage potential is produced thereacross and which functions as a gate bias voltage. In some instances, this bias voltage is sufficient to drive the turned off FET into conduction. Since the FET which should be nonconductive is, instead, conducting, the complementary symmetry is upset and the output signal applied to the load exhibits undesired distortion. Moreover, there is the possibility that the gate bias voltage may reach sufficient magnitudes whereby the FET is subjected to substantial damage, and may even be destroyed. This is especially pronounced when a signal of large amplitude is applied to the output stage.